1. Field of the Invention
The present invention concerns magnetic resonance imaging systems, and operating methods therefor, wherein radio-frequency (RF) pulses are designed for use in exciting nuclear spins in order to obtain magnetic resonance image data. The method also concerns a computerized processor programmed to design such RF pulses.
2. Description of the Prior Art
The spectral response of a spin system to a train of RF pulses may crucially define the content of the received MR signal and therefore image quality. Particularly if the MR application relies on a coherent and defined phase evolution of transversal magnetization between two successive RF pulses (as in TrueFISP or frequency selective RF pulses) spatial variations of the BO field, or equivalently, the resonance frequency, may lead to unwanted local signal behavior and image degradation. Examples of commonly used MRI methods where the detrimental effect of BO field inhomogeneity is particularly apparent include TrueFISP, chemical-shift selective RF pulses, and spectral-spatial RF pulses. For TrueFISP, off-resonance effects lead to well known dark stripe artifacts in regions where transversal magnetization accumulates a phase of (2n+1)*180° between two successive excitation pulses due to off-resonance (n=1,2,3, . . . ). For chemical shift selective RF pulses such as those for fat suppression in imaging or spectroscopy or water suppression pulses in spectroscopy, dephasing between sub-pulses due to off-resonance lead to incomplete excitation for frequencies to be excited and partial excitation of frequencies not to be excited.
Artifacts due to off-resonance in spectrally sensitive sequences or pulses are typically addressed by optimizing field adjustments due to so-called shim coils that produce linear and higher-order spatial field variations. This procedure, or shimming, while helpful, never fully removes the effects of undesired spatial variation in the static magnetic field, mostly since the field variations due to susceptibility variations in the subject are of substantially higher order than are available in commercial shim systems (which are often limited to 2nd order) and therefore fundamentally not amenable to shimming. Further hardware enhancements like higher-order shimming yields to expensive systems with high current demands. Other means to mitigate the detrimental effects of residual BO inhomogeneity are tailored to each specific application, and include, cases where the pulse or sub pulse spacing is not restricted to a specific timing, it might be optimized to reduce artifacts like in TrueFISP, where the repetition time is chosen as short as possible to minimize de-phasing time for off-resonance magnetization. Other examples of BO inhomogeneity mitigation in spectrally-selective pulses include 3D spectral-spatial pulses where the design offers a degree of control of the spectral passband as a function of spatial coordinates Morrell G, Macovski A. “Three-dimensional Spectral-Spatial Excitation.,” Magn. Reson. Med. 37(3): pgs 378-386; 1997. Mitigation of BO effects in parallel transmission (pTx) by incorporating off-resonance maps as design constraints in pulse optimization is described in Setsompop K, Wald L L, Alagappan V, Gagoski B A, Adalsteinsson E. “Magnitude Least Squares Optimization for Parallel Radio Frequency Excitation Design Demonstrated at 7 Tesla with Eight Channels,” Magn. Reson. Med. 59(4): pgs 908-915; 2008; and Setsompop K, Alagappan V, Gagoski B, Witzel T, Polimeni J, Potthast A, Hebrank F, Fontius U, Schmitt F, Wald L L, Adalsteinsson E. “Slice-Selective RF Pulses for In Vivo B1+ Inhomogeneity Mitigation at 7 Tesla Using Parallel RF Excitation with a 16− Element Coil,” Magn. Reson. Med. 60(6): pgs 1422-1432; 2008.
Moreover, current RF pulse design methods allow for exciting magnetization with spatially pre-defined magnitude and phase within the transversal plane. This is either achieved by a complex superposition of RF fields from multiple antennas with certain amplitude and phase relation (e.g. Ibrahim T S, Lee R, Baertlein A B, Kangarlu A, Robitaille P M L. “Application of Finite Difference Time Domain Method for the Design of Birdcage RF Head Coils Using Multi-Port Excitations,” Magn. Reson. Imaging 18: pgs 733-742; 2000) or by playing out gradient fields simultaneously to RF transmission (e.g. Pauly J, Nishimura D, Macovski A. “A K-space Analysis of Small Tip Angle Excitation,” JMR 81: pgs 43-56; 1989.) or a combination of both, which enables the acceleration of RF pulses with spatially tailored excitation patterns (e.g. Katscher U, Bornert P, Leussler C, van den Brink J S. “Transmit SENSE,” Magn. Reson. Med 49(1) pgs 144-150, 2003) through the reduced pulse duration.